I apologize for the length of my post. I was trying to give as much detail as possible. I hadn't actually checked back on this topic until today but I did want to let everyone know I have managed to get the interface working, just not quite in the configuration I wanted.
I had to leave my Netopia configured the same as it was previously - establishing the PPPoE connection and doing the default NAT, with its ethernet interface set up as a gateway. PF I set both interfaces to a static IP and the WAN pointed to the Netopia gateway. I've also left the automatic NAT rules on, not even messing with NAT. Everything works! I can see traffic going across the nice graph interface and have begun to try a couple of firewall rules.
It isn't quite the way I was hoping the configuration would be set up, but so far since it's working fine and I've been able to test blocking addresses locally, it seems to be doing the job. Many thanks.